1. Technical Field
The present invention relates to analytical separation technology and more specifically towards gas chromatography separation systems based on sol-gel stationary phases having improved performance characteristics.
2. Background Art
The introduction of an open tubular column by Golay (Golay, M. J. E., et al.) about three decades ago, has revolutionized the analytical capability of gas chromatography (hereinafter “GC”). More specifically, capillary GC has matured into a separation technique that is widely used in various fields of science and industry (Altgelt, K. H., et al.; Clement, R. E.; Berezkin, V. G., et al.; and Tebbett, I.). Capillary GC is a separation technique in which the vapor phase of a sample in a gaseous, mobile phase passes through a capillary tube whose inner walls contain a thin film of an adsorbing or absorbing medium (i.e., stationary phase). Because of differential interactions of the sample components with the stationary phase, the individual components of the sample move through the column with different velocities. This leads to the physical separation of the sample components into individual chromatographic zones as they move down the column with their characteristic velocities. The separated components are detected instrumentally as they are eluted from the column. Contemporary technology for the preparation of open tubular columns is time-consuming. It consists of three major, individually executed steps (Poole, C. F., et al.): capillary surface deactivation (Woolley, C. L. et al.), static coating (Bouche, J. et al.), and stationary phase immobilization (Blomberg L. G.). Involvement of multiple steps in conventional column technology increases the fabrication time and is likely to result in greater column-to-column variation. The column deactivation step is critically important for the GC separation of polar compounds that are prone to undergo adsorptive interactions (e.g., with the silanol groups on fused silica capillary inner walls). In conventional column technology, deactivation is usually carried out as a separate step, and involves chemical derivatization of the surface silanol groups. Various reagents have been used to chemically deactivate the surface silanol groups (de Nijs; R. C. M., et al.; Schomburg, G. et al.; Blomberg, L. et al.; and Lee, M. L. et al.). Effectiveness of these deactivation procedures greatly depends on the chemical structure and composition of the fused silica surface to which they are applied.
Of special importance are the concentration and mode of distribution of surface silanol groups. Because the fused silica capillary drawing process involves the use of high temperatures (˜2,000° C.), the silanol group concentration on the drawn capillary surface can initially be low due to the formation of siloxane bridges under high-temperature drawing conditions. During subsequent storage and handling, some of these siloxane bridges can undergo hydrolysis due to reaction with environmental moisture. Thus, depending on the post-drawing history, even the same batch of fused silica capillary can have different concentrations of the silanol groups that can also vary by the modes of their distribution on the surface.
Moreover, different degrees of reaction and adsorption activities are shown by different types of surface silanol groups (Lawrocki, J.). As a result, fused silica capillaries from different batches or even from the same batch but stored and/or handled under different conditions, cannot produce identical surface characteristics after being subjected to the same deactivation treatments. This makes surface deactivation a difficult procedure to reproduce. To overcome these difficulties, some researchers have used hydrothermal surface treatments to standardize silanol group concentrations and their distributions over the surface (Sumpter, S. R. et al.). This additional step however, makes the time consuming column making procedure even longer. Static coating is another time-consuming step in conventional column technology. A typical 30-m long column can require as much as ten hours or more for static coating. The duration of this step can vary depending on the length and diameter of the capillary, and the volatility of the solvent used.
To coat a column by the static coating technique, the fused silica capillary is filled with a stationary phase solution prepared in a low-boiling solvent. One end of the capillary is sealed using a high viscosity grease or by some other means (Abe, I. et al.), and the other end is connected to a vacuum pump. Under these conditions, the solvent begins to evaporate from the capillary end connected to the vacuum pump, leaving behind the stationary phase that becomes deposited on the capillary inner walls as a thin film. Stationary phase film of desired thickness could be obtained by using a coating solution of appropriate concentration that can be easily calculated through simple equations (Ettre, L. S. et al.).
In static coating, two major drawbacks are encountered. First, the technique is excessively time consuming, and not very suitable for automation. Second, the physically coated stationary phase film shows a pronounced tendency to rearrangements that can ultimately result in droplet formation due to Rayleigh instability (Bartle, K. D. et al.). Such a structural change in the coated films can serve as a cause for the deterioration or even complete loss of the column's separation capability.
To avoid these undesirable effects, static-coated stationary phase films need to be stabilized immediately after their coating. This is usually achieved by stationary phase immobilization through free radical cross-linking (Wright, B. W. et al.) that leads to the formation of chemical bridges between coated polymeric molecules of the stationary phase. In such an approach, stability of the coated film is achieved not through chemical bonding of the stationary phase molecules to the capillary walls, but mainly through an increase of their molecular size and consequently, through decrease of their solubility and vapor pressure.
Such an immobilization process has a number of drawbacks. First, polar stationary phases are difficult to immobilize by this technique (Yakabe, Y., et al.). Second, free radical cross-linking reactions are difficult to control to ensure the same degree of cross-linking in different columns with the same stationary phase. Third, cross-linking reactions can lead to significant changes in the polymer structure and chromatographic properties of the resulting immobilized polymer can significantly differ from those of the originally taken stationary phase (Blomberg L. G.). All these drawbacks add up to make column preparation by conventional techniques a task that is difficult to control and reproduce (Blomberg, L., et al.).
In order to overcome all of the above problems, a preparation of a GC capillary column including a tube structure and a deactivated surface-bonded sol-gel coating on a portion of the tube structure forming a stationary phase was disclosed and claimed in PCT Application PCT/US99/19113, published as WO 00/11463, to Malik et al. The invention disclosed therein is for a structure for forming a capillary tube, e.g., for gas chromatography, and a technique for forming such capillary tube. The capillary tube includes a tube structure and a deactivated surface-bonded sol-gel coating on a portion of the tube structure to form a stationary phase coating on that portion of the tube structure. The deactivated sol-gel stationary phase coating enables separation of analytes while minimizing adsorption of analytes on the separation column structure. This type of column was a significant advancement in the art, but it was recognized that certain improvements would greatly enhance the performance of the sol-gel coated column.
One area of improvement deals with baseline stability. A GC column is commonly operated under temperature-programmed conditions whereby the temperature of the column is increased with time. As the column temperature increases, the gas chromatography baseline rises because of column bleed caused due to the formation of volatile compounds from the stationary phase coating on the inner surface of the capillary column. In GC columns with polyslioxane-based stationary phases, the formation of volatile cyclic compounds is favored by the flexibility of the polysiloxane chains. One way to overcome or significantly reduce the column-bleeding problem is to reduce the flexibility of the polymeric structure of the GC stationary phase by incorporating phenyl rings in the polysiloxane backbone. This reduces the flexibility of polysiloxanes, and consequently, their ability to produce cyclic volatiles through rearrangements. The selection of the phenyl-containing reagent and the degree of substitution in the polysiloxane backbone are both critical, and care must be taken so that the stationary phase does not become too rigid. Otherwise, chromatographic properties of the polymer (especially the mass transfer properties) can be compromised. In an attempt to provide increased baseline stability, Mayer et al. used 1,4-bis(hydroxydimethylsilyl)benzene to incorporate a phenyl ring in the polydimethyldiphenylsiloxane structure by conducting its reaction with diphenylsilanediol at 110° C. for 48 hours. This non-sol-gel process however, is inconvenient for two reasons. First, the process is lengthy and carried out at elevated temperature. Second, the 1,4-bis(hydroxydimethylsiyl)benzene reagent used for the incorporation of the phenyl group provides a polymer structure where the phenyl ring is directly bonded to silicon atoms without any spacer groups and leads to a very rigid polymer affecting its mass transfer properties and chromatographic efficiency.
Accordingly, there is a need for an improved GC column having improved baseline stability, higher efficiency, and reduced conditioning time. Additionally, there is a need for a sol-gel GC column having desired stationary phase film thickness and improved retention characteristics that are capable of being fabricated into long columns. The present invention describes a sol-gel chemistry-based process that provides all of the above-mentioned desirable column characteristics through a simple procedure carried out under mild thermal conditions.